Field of the Invention
Embodiments of the present invention generally relate to a RF micro electromechanical system (MEMS) digital variable capacitor (DVC) and methods for manufacture thereof.
Description of the Related Art
MEMS DVCs are based on a moveable MEMS plate with a control-electrode above (i.e., pull-up electrode, pull-off electrode or PU-electrode) and below (i.e., a pull-in electrode, pull-down electrode or PD-electrode) the moveable MEMS plate, as shown schematically in FIG. 1. These electrodes are covered with a top and bottom dielectric layer. In addition there is an RF-electrode below the moveable MEMS element. Between the moveable plate and the RF-electrode there is a gap that is modulated by the voltages applied to either the PU or the PD-electrode with respect to the plate-electrode. These voltages result in electrostatic forces, which pull the MEMS element either up or down in contact to provide a stable minimum or maximum capacitance to the RF-electrode. In this way the capacitance from the moveable plate to the RF-electrode can be varied from a high capacitance state Cmax when pulled to the bottom (See FIG. 2) to a low capacitance state Cmin when pulled to the top (See FIG. 3).
FIG. 4 shows a more detailed cross-section view of a MEMS DVC device. The moveable plate consists of 2 plates (i.e., the bottom-plate and the top-plate) which are connected to each other via a plurality of posts. This combination of plates and posts creates a semi-rigid plate which is hard to bend. The plate is anchored to the substrate via a flexible leg-structure which allows relatively low operating voltages to operate the DVC device in the Cmin or the Cmax state.
When the lower plate is in contact with the dielectric layer overlying the RF electrode, the MEMS DVC is in the Cmax state. The distance between the lower plate and the RF electrode affects the capacitance. The RF electrode, if not substantially planar due to grain growth, negatively impacts the capacitance because asperites can form and cause an uneven topmost surface of the RF electrode, which in turn leads to an uneven dielectric layer formed thereover, which in turn leads to inconsistent capacitance in the Cmax state.
Therefore, there is a need in the art for a MEMS DVC and a method for manufacture that ensures a consistent capacitance.